The present invention relates generally to the formation of porous films. More specifically, the invention relates to porous materials and films comprising same having a low dielectric constant and methods for making same.
There is a continuing desire in the microelectronics industry to increase the circuit density in multilevel integrated circuit devices such as memory and logic chips to improve the operating speed and reduce power consumption. In order to continue to reduce the size of devices on integrated circuits, the requirements for preventing capacitive crosstalk between the different levels of metallization becomes increasingly important. These requirements can be summarized by the expression “RC”, whereby “R” is the resistance of the conductive line and “C” is the capacitance of the insulating dielectric interlayer. Capacitance “C” is inversely proportional to line spacing and proportional to the dielectric constant (k) of the interlayer dielectric (ILD). Such low dielectric materials are desirable for use, for example, as premetal dielectric layers or interlevel dialectric layers.
Undoped silica glass (SiO2), subsequently referred to herein as “USG”, has been long used in integrated circuits as a primary insulating material because of its relatively lower dielectric constant of approximately 4.0 compared to other inorganic materials. The industry has attempted to produce silica-based materials with lower dielectric constants by incorporating organics or other materials within the silicate lattice. For example, dielectric constants ranging from 2.7 to 3.5 can be achieved by incorporating terminal groups such as fluorine or methyl into the silicate lattice. These materials are typically deposited as dense films (density˜1.5 g/cm3) and integrated within the IC device using process steps similar to those for forming USG films.
Since the dielectric constant of air is nominally 1.0, yet another approach to reducing the dielectric constant of a material may be to introduce porosity or reducing the density of the material. A dielectric film when made porous may exhibit lower dielectric constants compared to a relatively denser film.
Porosity has been introduced in low dielectric materials through a variety of different means. For example, porosity may be introduced by decomposing part of the film resulting in a film having an increased porosity and a lower density. Additional fabrication steps may be required for producing porous films that ultimately add both time and energy to the fabrication process. Minimizing the time and energy required for fabrication of these films is desirable; thus discovering materials that can be processed easily, or alternative processes that minimize processing time, is highly advantageous.
A method used extensively in the literature for introducing porosity into a film is thermal annealing to decompose at least a portion of the film thereby creating pores and ultimately lowering the dielectric constant. In the annealing step, or curing step, the film is typically heated to decompose and/or remove volatile components and substantially cross-link the film. U.S. Pat. No. 6,312,793 describes a multiphasic material having a first phase consisting essentially of Si, C, O, and H, a second phase consisting essentially of C and H, and a multiplicity of pores. The material is heated to a temperature of at least 300° C. and for a time of at least 15 minutes to induce removal of one of the phases. Published patent application WO 00/02241 describes heating an alkoxysilane material at a temperature from 100 to 400° C. for a time of 1 to 10 minutes to induce formation of pores by removing the solvent contained therein. Published patent application WO 02/07191A2 describes heating a silica zeolite thin film to a temperature range of 350 to 550° C. for an unspecified amount of time to induce adsorbed material to leave the zeolitic framework thereby lowering the dielectric constant.
The majority of the aforementioned processes require curing steps at temperatures of 300° C. or higher and times of 30 minutes or longer. A primary concern in the production of a low dielectric film may be the overall thermal budget of the IC device. Consequently, various components of IC devices such as Cu metal lines can only be subjected to processing temperatures for short time periods before their performance deteriorates due to undesirable diffusion processes.
An alternative to the thermal anneal or curing step is the use of ultraviolet (“UV”) light in combination with an oxygen-containing atmosphere to create pores within the material and lower the dielectric constant. The references, Hozumi, A. et al. “Low Temperature Elimination of Organic Components from Mesostructured Organic-Inorganic Composite Films Using Vacuum Ultraviolet Light”, Chem. Mater. 2000 Vol. 12, pp. 3842-47 (“Hozumi I”) and Hozumi, A et al., “Micropattterned Silica Films with Ordered Nanopores Fabricated through Photocalcination”, NanoLetters 2001,1(8), pp. 395-399 (“Hozumi II”), describe removing a cetyltrimethylammonium chloride (CTAC) pore-former from a tetraethoxysilane (TEOS) film using ultraviolet (“VUV”) light (172 nm) in the presence of oxygen. The reference, Ouyang, M., “Conversion of Some Siloxane Polymers to Silicon Oxide by UV/Ozone Photochemical Processes”, Chem. Mater. 2000, 12(6), pp. 1591-96, describes using UV light ranging from 185 to 254 nm to generate ozone in situ to oxidize carbon side groups within poly(siloxane) films and form a SiO2 film. The reference, Clark, T., et al., “A New Preparation of UV-Ozone Treatment in the Preparation of Substrate-Supported Mesoporous Thin Films”, Chem. Mater. 2000, 12(12), pp. 3879-3884, describes using UV light having a wavelength between 187 and 254 nm to produce ozone and atomic oxygen to remove organic species within a TEOS film. These techniques, unfortunately, may adversely effect the resultant film by chemically modifying the bonds that remain within the material. For example, exposure of these materials to an oxidizing atmosphere can result in the oxidation of the C atoms contained therein which has an adverse effect on the dielectric properties of the material.
U.S. Pat. No. 6,284,500 describes using UV light in the 230 to 350 nm wavelength range to photoinitiate cross-linking within an organic polymer film formed by CVD or an organosilsiquoxane film formed by spin-on deposition to improve the adhesion and mechanical properties of the film. The ′500 patent teaches that a thermal annealing step may be used to stabilize the cross-linked film.
Published U.S. patent application Ser. No. 2003/054115 (the ′115 application) teaches UV curing a porous dielectric material produced by CVD or spin-on deposition methods to produce a UV-cured porous dielectric material having an improved modulus and comparable dielectric constant. The ′115 application demonstrates that UV exposure in an O2 atmosphere is more effective than UV exposure in a N2 atmosphere. However, the ′115 application also teaches that the UV cure can generate a notable amount of polar species in the porous dielectric materials. Further, the ′115 application states that “in all cases a subsequent or possibly concomitant anneal step is necessary in order to remove the Si—OH bonds which are typically generated during the UV curing process.”
U.S. Pat. No. 6,566,278 teaches densifying a carbon-doped, silicon oxide (SiCxOy) film by exposing the film to UV radiation. The carbon-doped silicon oxide film is deposited via chemical vapor deposition of an oxygen-supplying gas and an organosilane silicon supplying gas. The film is then exposed to UV radiation generated from an excited gas species such as xenon, mercury, deuterium, or KrCI2.
Accordingly, there is a need in the art to provide an improved method to produce low density and porous materials. There is also a need to provide a process for preparing a low dielectric film that does not inhibit the pore formation process. There is a further need to provide a method that is selective in removing certain material from a film. Due to thermal budget concerns, there is an additional need for a low temperature treatment for the production of porous films for low dielectric constant materials for integrated circuits.
All references cited herein are incorporated herein by reference in their entirety.